Multi-layer systems have been utilized to coat automobiles for a number of years, but the early development of these systems necessarily employed organic solvents. As environmental regulations became more stringent, and the cost of organic solvents rose, organic-borne basecoat systems became less desirable. The recent research emphasis in the area of multi-layer systems, especially basecoat systems has focused on the development of water-borne systems for multi-layer coatings.
The shift from organic solvents to water for dispersing and applying resins in multi-layer systems solved many of the environmental and cost problems associated with the use of organic solvents. Water-borne systems, however, have resulted in other problems.
The application of a multi-layer coating to an automobile body, for example, is greatly facilitated by a system that provides for drying of solvent after the application of a coating. This is typically accomplished by flashing underlying coatings such as the basecoat prior to application of the topcoat layer(s). In a flashing operation, the coating is exposed to air, optionally at elevated temperature, for a certain period of time. The time and temperature of the exposure is usually not sufficient to provide any significant curing to the basecoat, but is sufficient to drive enough solvent from the coating to avoid problems caused by excess solvent being trapped under the topcoat layer(s).
By facilitating drying, the time between coatings would be diminished, resulting in manufacturing efficiencies and lower energy costs. In addition, there would be a reduced need for a cool-down zone after drying which would further the manufacturing efficiencies. However, when the solvent for a coating composition is water, the amount of time required for driving off the water is often significantly greater than for organic solvent systems. It is thus desireable to produce a waterborne basecoat composition that could be dried in a period of time short enough to maintain manufacturing efficiency.